This invention relates to a thin, flat electrical cell and battery construction, and more particularly to novel apparatus for venting such cells and batteries.
Presently flat multicell batteries are utilized as power supplies to actuate motor-driven film processing means and optical adjustment apparatus including shutter control apparatus in automated cameras adapted to effect sequential exposure and development of photographic film. Representative of such cameras are those described, for example, in U.S. Pat. Nos. 3,750,551, 3,744,385, 3,731,608, and 3,714,879. For such a photographic application, the battery should be thin and compact and should have a relatively low impedance to produce a high output current over a time period corresponding to a reliable shelf life.
Batteries of the kind described are composed of cells having a plurality of thin, flat layers including a positive electrode, a negative electrode and an electrolyte-containing layer, which layer itself can comprise one or more layers. In multicell batteries, the cells are separated by an electrically conductive layer such as a conductive plastic material, which functions also to prevent electrolyte migration between cells. The electrically conductive layer is positioned between adjacent cells to contact the negative electrode of one cell and the positive electrode of the next adjacent cell. The periphery of the battery construction is sealed to minimize water vapor loss from the cells and to minimize contamination of the cell by gases, particularly oxygen, in the ambient atmosphere.
A major problem associated with these batteries is that the materials comprising the electrolyte cause formation of hydrogen-rich gas within the battery. If the gas pressure within the battery is not controlled, the battery will expand and the layers within the battery become separated, thereby seriously reducing electrical conductivity between the cells and the current output of the battery. In addition, gas formation increases the battery thickness so that it cannot be used in the camera for which it is designed. Furthermore, the increased gas pressure reduces the integrity of the battery seals, thus rendering the battery inoperative within a short time.
A variety of battery venting means presently are available for selectively removing gas produced in the battery while preventing liquid water removal therefrom and while preventing invasion of the outside atmosphere into the battery.
U.S. Pat. No. 3,870,566 discloses a venting system for a thin, flat battery comprising at least one solid or hollow fiber formed of a material pervious to the gas produced in the battery. The interior of the hollow fibers communicate with the atmosphere, and the fibers are positioned adjacent a liquid impervious, hydrogen gas-pervious plastic layer which separates the hollow fibers from contact with either the electrodes or the electrolyte.
U.S. Pat. No. 3,647,557 discloses a battery which includes a hollow fiber formed from a liquid-impermeable plastic material that is pervious to the gas produced in the battery. The fiber extends through the battery wall into the battery interior either in contact with the liquid electrolyte or into the space above the electrolyte. The exterior of the fiber wall is sealed at the points where it extends through the battery wall and is formed so that its interior does not communicate directly with the battery interior.
U.S. Pat. No. 3,741,813 describes a battery which includes a nonconductive, gas-pervious, liquid-impervious outer wall to house the electrodes and the electrolyte. A conductive liquid-impervious, gas-pervious membrane is positioned adjacent the wall and is sealed around the perimeter of a hole in the wall. The membrane has at least one hole that communicates the wall to the battery interior. The hole in the wall is offset from the membrane holes to expose the conductive membrane to the atmosphere.
U.S. Pat. No. 3,741,812 shows a battery construction similar to that shown in U.S. Pat. No. 3,741,813 except that the outer wall is electrically conductive and contains a hole while the membrane sealed to the hole and located between the outer wall and the battery interior is nonconductive and does not contain a hole. In the batteries disclosed in these patents, the holes and the gas-pervious walls and membranes prevent gas build-up within the battery and permit the introduction of oxygen into the battery where the positive electrode comprises an oxidizing agent.
U.S. Pat. No. 2,632,784 discloses a battery vent formed of a flat tape comprising woven fibers backed with a gas-permeable adhesive which is sealed longitudinally to a stack of cells forming the battery wherein at least one end of the tape is open to the atmosphere. U.S. Pat. No. 3,081,371 discloses a venting means for a cell comprising a hole in a wrapping film for the electrodes positioned to minimize electrolyte leakage.
Venting mechanisms such as those described above typically leave something to be desired in one or both of the desired qualities of reproducibility and selectivity. For example, it is very difficult to install a fine hollow tube in a battery without closing or partly closing the tube, leading to underventing, or to overventing if a larger tube is used in cases where it is not closed during assembly. If a plurality of such tubes are used to make sure that at least one is not closed, the problem is exacerbated because the tubes become smaller, and the chances of encountering overventing or underventing are not improved.
The objects of this invention are to improve the reliability and to increase the selectivity of vents for laminar batteries.
Briefly, the above and other objects of the invention are attained by a novel vent construction comprising one or more plastic tubes each containing packing fibers along the length of the hollow interior of the tube. The packing fibers provide mechanical stability to the tube so that emplacement of the tube in the battery can be made without collapsing the tube. The walls of the tube are permeable to hydrogen, and impermeable to liquids. Since the tube is in a noncollapsed state in the battery, and because the gas and water vapor permeability characteristics of the compositions utilized to form the tube can be predicted with accuracy, the battery venting system of the invention provides a significant improvement over prior art battery venting systems. Specifically greatly increased control is provided in balancing the transport of the gases generated in the battery, the outside atmosphere, and water vapor, through the venting system in a manner so that battery life can be greatly extended. In addition, a venting system in accordance with the invention provides excellent selectivity without critical demands on the materials employed in the vents. In particular, the walls of a fiber packed tube used as a vent serve as the primary diffusion impedance to the diffusion of hydrogen, whereas the elongated passage through the tube around the packing fibers serves as an additional impedance to the diffusion of water vapor. Both the walls of the hollow tube and the elongated passage through it are significant oxygen diffusion barriers, although the tube wall is presumably the limiting impedance to oxygen flow. Thus, the material for the hollow tube, and its area and thickness can be selected primarily on the basis of hydrogen permeability and its ability to participate in the battery seals, without great concern for permeability to water vapor. For example, polyvinyl chloride, while relatively permeable to water vapor, is well suited to the practice of the invention.
The fiber-filled tube is positioned in the battery so that the gases generated in the battery pass through the walls of the tube either directly or indirectly by being passed first through a gas permeable, liquid impermeable layer in the battery and then through the tube wall. When the tube terminates in a wet region of the battery, the end of the tube within the battery is closed to prevent direct communication of the fiber packed interior of the tube with the interior of the battery. In any event, at least one end of the fiber-filled tube is open to the atmosphere exterior of the battery. The packed tube can be positioned so that it extends across a portion of or over the entire battery width or length.